ctdb: use mlockall, cautiously

ctdb: use mlockall, cautiously

We don't want ctdb stalling due to paging; this can be far worse than
scheduling delays.  But if we simply do mlockall(MCL_FUTURE), it
increases the risk that mmap (ie. tdb open) or malloc will fail,
causing us to abort.

This patch is a compromise: we mlock all current pages (including
10k of future stack for expansion) and then relock when a client
asks us to open a TDB.  We warn, but don't exit, if it fails.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>